How to get a MovieClipSequence Clip File via Python
The active 'clip name' is the only real reference at the moment so avoid 'renaming' in VSE:

import bpy

seq = bpy.data.scenes['Scene'].sequence_editor
strip_name = seq.active_strip.name

print (bpy.data.movieclips[strip_name].filepath)


Console output: /home/user/videos/file_name.mp4



Otherwise (although not ideal) you could iterate through bpy.data.movieclips and compare the duration of the 'active strip' with 'all other' clips:

import bpy

seq = bpy.data.scenes['Scene'].sequence_editor
strip = seq.active_strip
strip_duration = strip.frame_duration

for i in bpy.data.movieclips:
    if (i.frame_duration == strip_duration):
        print (i.filepath)